The present invention relates to a servo system which carries out a feedback control of positions, speeds and so on of machine tools and so on.
In recent years, a serial multi-drop method is generally used to connect a controller, such as a numeric control apparatus or a motion controller, for controlling a plural number of shafts to servo drive units. FIG. 5 is a wiring diagram of an exemplified system utilizing a drive-unit-built-in servo motors in which a servo drive unit is incorporated. The figure shows that a controller 1 and a drive-unit-built-in servo motor 5a are connected by a cable 4a and a second drive-unit-built-in servo motor 5b and the drive-unit-built-in servo motor 5a are connected by a cable 4b. A third one and a succeeding one are connected in series in the same way, and the drive-unit-built-in servo motors 5a, 5b, 5c and 5d respectively comprise rotary switches 19a, 19b, 19c and 19d for shaft number identification.
FIG. 6 shows a detailed signal diagram of the exemplified system. The figure shows that the send data from the controller 1 are transmitted as serial data for all shafts through the identical lines. From the serial data received, the servo drive units in the drive-unit-built-in servo motors respectively select data which correspond to their respective shaft numbers and drive servo motors based on the data. (FIG. 6 does not show detailed circuits, servo motors and other components beyond line drivers and line receivers of the servo drive units inside the drive-unit-built-in servo motors. The drive-unit-built-in servo motor 5d is omitted in the figure.) At this moment, the shaft numbers are identified on the basis of the settings of hardware such as rotary switches 19 provided inside the servo drive units.
The signals from the drive-unit-built-in servo motors are output to the identical line by time division and transmitted to the controller 1. An emergency stop (EMG) signal is output from the controller 1 and transmitted to the first drive-unit-built-in servo motor 5a. The drive-unit-built-in servo motor 5a transmits logical sum of the received EMG signal and an alarm signal of its own to the second drive-unit-built-in servo motor 5b. The processing proceeds in the same way till the final shaft and the output from the drive-unit-built-in servo motor of the last shaft is transmitted to the controller 1.
Since distances between the drive-unit-built-in servo motors in the exemplified system shown in FIGS. 5 and 6 are long, a total cable length is so extensive that there has been a fear that communication on the side of the last shaft may be disabled by such as an influence of reflected waves. In the exemplified system, since each of the drive-unit-built-in servo motors is connected to two cables, a large space needed for the connector on the side of the servo motors and a complicated cable arrangement have been the problems.
Another problem has been that, since the shaft numbers are set by setting such hardware as the rotary switches 19 attached to the drive-unit-built-in servo motors, alteration in the setting is not always easy, depending on the location of the servo motors. Still another problem has been that the structure becomes complicated since such hardware as the rotary switches 19 needs to have an anti-splash structure.